A high-carbon pearlitic steel has been used as a rail material of a railroad due to its excellent abrasion-resistant. However, there has been a problem in that the ductility or toughness is low since the content of carbon is extremely high.
For example, with regard to an ordinary carbon steel rail containing carbon at an amount of 0.6 to 0.7% by mass as shown in Non-Patent Document 1, the impact value at room temperature is in a range of about 12 to 18 J/cm2 which is measured by Charpy impact testing of JIS No. 3 U-notch specimen. In the case where this ordinary carbon steel rail is used in a low-temperature region, such as a cold weather region, there has been a problem in that brittle fractures occur due to fine initial defects or fatigue cracking.
In addition, the carbon amount in a rail steel has been further increased in order to improve the wear resistance in recent years; and therefore, there has been a problem in that the ductility and the toughness are further degraded due to the increased carbon amount.
It is generally known that refinement of pearlite structure (pearlite block size) which is specifically, refinement of austenite grains before pearlite transformation or refinement of the pearlite structure during pearlite transformation is effective for improving the ductility and the toughness of pearlitic steels.
Examples of a method of refining the austenite grains include lowering of the reheating temperature during reheating of a bloom for rail rolling, lowering of the rolling temperature during hot rolling, and increasing of the reduction of cross-sectional area during hot rolling.
However, in a process of manufacturing a rail, there is a problem in that, even in the case where the refinement of the austenite grains immediately after rolling can be achieved by the above-mentioned method, the grains grow until a thermal treatment starts; and consequently, the ductility is degraded.
In addition, transformation acceleration from the inside of austenite grains is carried out by utilizing transformation nuclei in order to achieve the refinement of the pearlite structure during pearlite transformation (for example, Patent Document 1).
However, with regard to the pearlite transformation from the inside of austenite grains by utilizing transformation nuclei, there are problems in that it is difficult to control the amount of transformation nuclei, and the pearlite transformation from the inside of the grains is not stable. As a result, sufficient refinement of the pearlite structure may not be achieved.
In view of the above-mentioned various problems, a method of refining pearlite structure has been applied in order to fundamentally improve the ductility and the toughness of rails having pearlite structure, and this method includes: reheating at low temperatures after rolling of a rail; and performing accelerated cooling thereafter to conduct pearlite transformation; and thereby, the pearlite structure is refined (for example, Patent Document 2).
However, the carbon amount in a rail has been increased in order to improve the wear resistance in recent years. Therefore, there has been a problem in that coarse carbides are not completely melted and remain in austenite grains during the above-mentioned reheating treatment at low temperatures; and thereby, the ductility and the toughness of the pearlite structure after the accelerated cooling are degraded. In addition, since this method includes reheating, there has been a problem of economic efficiencies, such as high manufacturing costs, low productivity, and the like.
In view of these circumstances, a pearlitic rail having improved ductility and a production method thereof were developed (Patent Documents 3 and 4). In the pearlitic rail, pinning effect due to precipitates is utilized; and thereby, the growth of austenite grains is suppressed, and pearlite blocks are refined. As a result, the ductility is improved.
However, in the case of the pearlitic rail and the production method thereof according to Patent Documents 3 and 4, it is necessary to perform reheating at low temperatures in order to finely disperse AlN; and therefore, there are problems in that it is difficult to secure the rolling formability, and the ductility is degraded due to generation of pro-eutectoid cementite in the inner of the head portion.